(1) Field of the Invention
This invention relates to a substrate heat treatment apparatus for heat-treating substrates such as semiconductor wafers, glass substrates for liquid crystal displays, glass substrates for photomasks and substrates for optical disks (hereinafter simply called “substrates”). More particularly, the invention relates to a technique for heat-treating a substrate as sucked in a position slightly spaced from a heat-treating plate.
(2) Description of the Related Art
With an increasingly fine line width of patterns formed on substrates today, the requirements for line width uniformity have become stringent, which has led to a strong demand for temperature uniformity in baking treatment in photolithography, especially in baking treatment after exposure (PEB: Post Exposure Bake). However, with enlarged substrate sizes, increased curvatures of substrates take place in the semiconductor manufacturing process. It is difficult to satisfy the requirements for temperature uniformity in a proximity heating mode that heats each substrate only by placing the substrate as separated by a minute space from a heat-treating plate.
Thus, a suction bake mode has been proposed in order to perform uniform heat treatment even for curved substrates. This type of apparatus includes a heat-treating plate with a heater, support elements and a sealer arranged on the upper surface of the heat-treating plate, and exhaust bores formed in the upper surface of the heat-treating plate (as disclosed in Japanese Unexamined Patent Publication H10-284360 (1998), for example). With this apparatus, the sealer seals lateral areas of a space formed between a substrate supported by the support elements and the heat-treating plate, and the substrate is sucked by exhausting gas from the space through the exhaust bores. By sucking the substrate, any curvature of the substrate is eliminated whereby the substrate can be heated uniformly.
(I)
The conventional apparatus noted above has the following drawback.
With the conventional apparatus, the substrate under suction sags between each pair of support elements. This aspect will particularly be described with reference to FIG. 1. FIG. 1 is a fragmentary sectional view of a heat-treating plate 51. As seen, a substrate or wafer W is supported by a plurality of support elements 53. A minute space “ms” between the wafer W and heat-treating plate 51 is sealed by a sealer 55. The minute space “ms” is placed under negative pressure by drawing gas therefrom through exhaust bores not shown. At this time, the wafer W sags between each adjacent pair of support elements 53 to be curved toward the heat-treating plate 51 (as shown in solid lines). The difference between maximum and minimum of separation of the wafer W and heat-treating plate 51 is called herein an amount of bending b. In FIG. 1, the dotted line shows the wafer W with the amount of bending at 0 μm.
The amount of bending may be reduced by arranging an increased number of support elements 53 to shorten the intervals therebetween. However, the shorter the intervals are between the support elements 53, the more support elements 53 contact the wafer W. To worsen the situation, the increased contact parts will become sources of particles.
(II)
The conventional apparatus noted above has also the following drawback.
In a photolithographic process, for example, when a sensitizing agent (resist) is applied to a substrate surface, the resist will flow around the end surface to the back surface of the substrate, and adheres to such parts. Consequently, in heating treatment before exposure (i.e. prebake treatment) heating treatment after exposure (PEB: Post Exposure Bake) and heating treatment after development (postbake treatment), when the sealer contacts the edges of the substrate, the resist will be transferred from the substrate to the sealer. This will bring about an inconvenience of causing cross contamination. Such an inconvenience is not limited to the case of sensitizing agent application in the photolithographic process, but can occur also with substrate treatment using other chemicals.